What does the echo train length (ETL) indicate in a fast spin echo sequence?

The echo train length (ETL) in a fast spin echo (FSE) sequence specifically refers to the number of 180-degree refocusing pulses that are applied during the sequence. Each refocusing pulse generates an echo, enabling the system to collect multiple echoes in a single acquisition. This is a significant aspect of fast spin echo imaging because it allows for quicker processing of data and reduces scan times compared to conventional spin echo sequences, which typically require a single echo per excitation.

By applying multiple refocusing pulses, each separated by a defined interval, the ETL influences both the image quality and the time efficiency of the MRI scan. A longer ETL can improve signal-to-noise ratio (SNR) and image resolution by providing more data points, but it may also increase the risk of T2 weighting effects, potentially compromising image quality if not managed properly.

Thus, the correct answer reflects the fundamental mechanism of capturing multiple echoes via the application of 180-degree refocusing pulses in a fast spin echo sequence, which is critical to the functional performance of this MRI technique.